1. Field of the Invention
The present invention relates to a method of correcting chromatic and spherical aberrations in a charged-particle beam instrument, such as an electron beam instrument (e.g., a scanning electron microscope) or an ion beam instrument (e.g., an ion microprobe). The invention also relates to such a charged-particle beam instrument.
2. Description of Related Art
The following non-patent references appear to be relevant to the present invention: (1) H. Rose, Optik 33, Heft 1, pages 1-24 (1971); (2) J. Zach, Optik 83, No. 1, pages 30-40 (1989); (3) J. Zach and M. Haider, Nucl. Instr. and Meth. In Phys. Res. A 363, pages 316-325 (1995); and (4) M. Haider et al., Optik 63, No. 1, pages 9-23 (1982).
In scanning electron microscopes and transmission electron microscopes, an aberration corrector is incorporated in the electron optics in order to achieve high-resolution imaging or enhance the probe current density. In a proposed system, this aberration corrector is fitted with multipole units each having twelve pole elements. Chromatic aberration is corrected by a combination of an electrostatic quadrupole operation mode and a magnetic quadrupole operation mode. Spherical aberration is corrected by an octupole operation mode using three or four stages. The principle is introduced in detail in the above-cited references 1-3.
The principle of the above-described aberration corrector is briefly described with reference to FIG. 1, where the aberration corrector, indicated by C, is disposed ahead of an objective lens 7. The corrector C has four stages of multipole units 51, 52, 53, and 54 each having twelve pole elements. Electric potentials 1′, 2′, 3′, and 4′ for activating the electrostatic quadrupole components are applied to the multipole units 51, 52, 53, and 54, respectively. Electric currents 5′ and 6′ for activating magnetic quadrupole components are applied to the second and third stages of multipole elements 52 and 53, respectively, to produce a magnetic potential distribution analogous with the electric potential distribution produced by the electrostatic quadrupole component-activating electric potentials 2 and 3 and to produce magnetic fields superimposed with electric fields. Thus, magnetic fields superimposed with the electric fields are set up. Furthermore, electrostatic octupole component-activating electric potentials 11′, 12′, 13′, and 14′ are applied to the multipole units 51, 52, 53, and 54, respectively, to produce electric fields superimposed with the electric fields developed by the quadrupole element-activating electric potentials 1′, 2′, 3′, 4′.
In actual instrumentation, electric potentials (acting as a deflector for axial alignment) for activating dipole elements and electric potentials (acting to correct second-order aperture aberrations) for activating hexapole elements are superimposed on the above-described electric fields produced by the potentials for activating the quadrupole components and octupole components. Since dipole and hexapole potentials are minimally directly associated with aberration correction for which the present invention is intended, their detail description is omitted.
In the configuration of FIG. 1, a beam of charged particles enters from the left side as viewed in the figure. The four stages of multipole units 51, 52, 53, 54 and the objective lens 7 create a reference orbit for the beam. This beam is brought to a focus on a specimen surface 20. In FIG. 1, orbits Rx and Ry in the X- and Y-directions, respectively, of the beam of particles are both drawn schematically on the same plane.
The reference orbit can be understood as follows. As a paraxial orbit that can be taken as an orbit in a case where there is no aberration, the orbit Ry in the Y-direction is caused to pass through the center of the multipole unit 52 by the multipole unit 51. The orbit Rx in the X-direction is made to pass through the center of the multipole unit 53 by the multipole unit 52. Finally, the beam of charged particles is brought to a focus on the specimen surface by the multipole units 53 and 54 and by the objective lens 7. The electric potentials 1′, 2′, 3′, and 4′ for activating electrostatic quadrupole components are applied to the multipole units 51, 52, 53, and 54, respectively. In practice, these need to be adjusted mutually for complete focusing. At this time, the aforementioned dipole element-activating potentials are used for axial alignment.
Referring more particularly to FIG. 1, the beam of charged particles in the orbit Rx in the X-direction is defocused by the multipole unit 51 acting like a concave lens. Then, the beam is focused by the multipole unit 52 acting like a convex lens so that the beam passes through the center of the multipole unit 53. Then, the beam is focused by the multipole unit 54 and directed toward the objective lens 7. Meanwhile, the beam of charged particles in the orbit Ry in the Y-direction is focused by the multipole unit 51 and made to pass through the center of the multipole unit 52. Then, the beam is focused by the multipole unit 53. Finally, it is defocused by the multipole unit 54 and directed toward the objective lens 7. In this way, the defocusing action of the multipole unit 51 acting on the X-direction orbit Rx and the defocusing action of the multipole unit 54 acting on the Y-direction orbit Ry are combined. The resulting action can act like a single concave or convex lens.
Chromatic aberration correction made by the aberration corrector C is now described. To correct chromatic aberration by the system shown in FIG. 1, the potential φq2 [V] for activating electrostatic quadrupole components and the magnetic excitation J2 [AT] (or magnetic potential) for activating magnetic quadrupole components are adjusted such that the reference orbit is not affected. The whole lens system acts to correct the X-direction chromatic aberration to zero. Similarly, the potential φq3 [V] for activating electrostatic quadrupole components and the magnetic excitation J3 [AT] for activating magnetic quadrupole components are adjusted such that the reference orbit is not affected. The whole lens system acts to correct the Y-direction chromatic aberration to zero.
Correction of spherical aberration (correction of the third-order aperture aberrations) is next described. Where spherical aberration is corrected, X- and Y-direction chromatic aberrations are corrected. Then, the X-direction spherical aberration in the whole lens system is corrected to zero by the potential φ02 [V] for activating electrostatic octupole components. The Y-direction spherical aberration is corrected to zero by the potential φ03 [] for activating electrostatic octupole components.
Then, the spherical aberration in the combined direction of the X- and Y-directions is corrected to zero by the electrostatic octupole component-activating potentials 11 and/or 14. In practice, repeated mutual adjustments are necessary. With respect to superimposition of the potentials and magnetic excitations at quadrupole and octupole components, the potential or excitation applied to each pole of a single dodecapole (twelve-pole) element is varied to synthesize dipole, quadrupole, hexapole, octupole, etc. This method has been put into practical use and introduced, for example, in the above-cited reference 4.
In particular, in an electrostatic design, a final stage of power supplies An (n=1, 2, . . . 12) capable of supplying voltages to twelve electrodes Un (n=1, 2, . . . , 12) independently is connected as shown in FIG. 2. Where a quadrupole field is produced, the output voltages from a quadrupole power supply 10 are supplied to the final-stage power supplies An to obtain a field close to an ideal quadrupole field. If it is assumed that the output voltages from the final-stage power supplies An are proportional to the output voltages from the quadrupole power supply 10, the ratio of the output voltages from the quadrupole power supply 10 assumes a value as given in the above-cited reference 4 above. Where an octupole field is created to be superimposed on this quadrupole field, the output voltages from an octupole power supply 18 are added to the output voltages from the quadrupole power supply 10 and supplied to the final-stage power supplies An to obtain a field close to an ideal octupole field. Similarly, a field on which a multipole field produced by a 2n-pole element (n=1, 2, . . . , 6) is superimposed is obtained using the single dodecapole element.
In a magnetic design, a final stage of power supplies Bn (n=1, 2, . . . , 12) capable of supplying excitation currents to the coils on twelve magnets Wn (n=1, 2, . . . , 12) independently is connected as shown in FIG. 3. Where a quadrupole magnetic field is created, the output voltages from a quadrupole magnetic-field power supply 15 are supplied to the power supplies Bn to produce a field close to an ideal quadrupole magnetic field. If it is assumed that the output currents from the final-stage power supplies Bn are proportional to the output voltages from the quadrupole magnetic-field power supply 15, the ratio of the output voltages from the power supply 15 assumes a magnetic excitation ratio as given in the above-cited reference 4 above. Superimposition of multipole fields other than a quadrupole magnetic field is not explained herein. However, a multipole magnetic field can be superimposed in the same way as in the electrostatic design, by adding voltages for the multipole field to the input voltages to the final-stage power supplies Bn. A yoke for magnetically connecting the outside portions of the magnets Wn is omitted in FIG. 3.
Where electrostatic and magnetic designs are superimposed, a conductive magnetic material may be used so that the magnets Wn can act also as the electrodes Un. In this case, the coils on the magnets are positioned to be electrically isolated from the electrodes.
In the description given below, the 2n-pole elements are treated as if they were stacked on top of each other to simplify the explanation. In practice, superimposition of multipole fields on a single dodecapole element is achieved by adding voltage signals as mentioned previously.
After the end of correction of chromatic aberration, it may be necessary to correct the second-order aperture aberration by means of three or four stages of hexapole elements before correction of spherical aberration is performed. This correction is made in the same procedure as in the aforementioned correction of spherical aberration. This second-order aperture aberration occurs depending on the mechanical accuracy of the aberration corrector. Normally, the amount of correction is small, and this aberration affects higher-order aberrations only a little within the scope of the present aberration corrector. The second-order aperture aberration is corrected within the aberration corrector. So, if the resultant magnification (described later) of the aberration corrector and the objective lens is varied, higher-order aberrations are affected little, though the resultant magnification is important in aberration correction. For this reason, description of the correction of the second-order aperture aberration is omitted herein.
In the description given below, an electric potential φ (or a voltage) represents a plus-side value in a normal array of multipole elements shown in FIGS. 4(a) and 4(b). Similarly, a magnetic excitation J in magnetic multipole elements represents a plus-side value [AT].
The aforementioned theory of aberration correction and the results of actually performed experiments demonstrate that chromatic and spherical aberrations are almost completely corrected. This proves the excellence of the aberration correction system described above. From a point of view of practicability, however, it can be said that sufficient consideration has not been given to the stability of the aberration correction system and to the range of the applied voltage and even to the optimum conditions. For example, the following problems have arisen.
First, where an aberration-correcting electric potential proportional to the accelerating voltage is used as shown in the prior-art example, i.e., in a case where the aberration-correcting potential applied to each pole element is made to vary in proportion to the accelerating voltage of the electron beam, if the accelerating voltage is set to a lower value, the noise component of the voltage or current used in the aberration corrector produces a greater effect.
Secondly, where an aberration-correcting potential proportional to the accelerating voltage is used as in the prior art example, if a large correcting potential is used at low accelerating voltages to reduce the effect of the noise component of the current, the withstand voltage of the aberration corrector at high accelerating voltages presents problems.
Thirdly, where an aberration-correcting potential proportional to the accelerating voltage is used as in the prior art example, if the instrument is made immune to noise components at low accelerating voltages, it is necessary to reduce the amount of noise component of the voltage or current of the power supply to a practical value. If this is achieved, the instrument is expensive to make.
Fourthly, if the resultant magnification MR of the aberration corrector and objective lens is adjusted to maintain constant the excitation current through the magnetic quadrupole components for correcting chromatic aberration, the aberration-correcting potential is non-relativistically in proportion to the square root of the accelerating voltage. This alleviates the first through third problems described above. However, where the range of the variable accelerating voltage is wide, the spherical aberration-correcting potential becomes too large at low accelerating voltages. As a result, an expensive power supply may be necessary.
Fiftly, complex data for correcting spherical aberration has been necessary for each value of the accelerating voltage and for each value of the working distance.